Fuse plug



Feb. 18, 1941. L. LUDWIG 2,232,463

FUSE PLUG Filed Feb. 15, 1 40 RNEY imli miq Patented Feb. 18, 1941 UNITED STATES PATENT OFFICE 4 Claims.

My invention relates to electrical protective devices and it is the object of my invention to provide a protector for electric circuits and which will also protect apparatus served thereby from damage arising from relatively small overloads which may become harmful by reason of their sustained duration, as well as from heavy overloads and short circuits, but that will not operate or open the circuit on harmless overloads, overloads having a magnitude or duration so small that no injury could result therefrom.

It is well known that electric motors, particularly alternating current motors, when started draw currents very much in excess of the currents required to keep them running under full load; for example a motor drawing five amperes when running at full load might require a starting current of from 12 to 40 amperes, depending on the type of motor and the load under which it is started. While it is true that the duration of these starting currents is relatively short, yet the time lag of the ordinary electric fuse in the small sizes is still shorter.

As a result, if an attempt is made to protect a small motor with the ordinary fuse, it will be found that a 5 or 6 ampere fuse on a 5 ampere motor will blow often when the motor is started.

A larger size fuse must therefore be used, such as a 10 ampere fuse. In such a case it is clear that if the motor is overloaded 100% the fuse still will not blow and as a result the motor will be damaged, if continuously overloaded.

Thermal overload devices having an operating time lag greater than that of the ordinary type of fuse have been used heretofore, but they have not anticipated all of the requirements of delayed action fuses.

In my invention I provide a device which embodies within itself the desirable characteristics of both thermal cutout and fuse, and it is a further object of my invention to provide such a device in an enclosure of a standard size or sizes, so that it may be used wherever an ordinary fuse can be used and so that it may be used as a replacement unit for ordinary fuses.

It is another object of my invention to provide a delayed action fuse in which the operating elements are positively connected in circuit relationship to avoid undue heating and to avoid sparking or radio interference.

Still another object is to provide a. delayed action fuse of simple construction and of 10w manufacturing cost, that is, at a cost closely approximating the manufacturing cost of ordinary fuses.

More especially it is an object of the invention to provide a fuse that meets the following Underwriters Laboratories tests:

1. Temperature test-In an ambient of 24 C. 5 the temperature of the exterior of the fuse enclosure shall not rise more than C. above that of the ambient when the fuse is carrying per cent current. 10

2. Overload blowing-In an ambient temperature between 18 and 32 C. fuses starting cold shall blow on per cent current within one hour, and on 200 per cent current within two minutes, without causing casings of combustible 15 material to char.

3. Short-circuit test.--When one fuse is blown at a time on a system of the voltage at which the fuse is rated, the fuse shall remain intact, and shall open the circuit without emitting sufficient 20 flame or molten metal to ignite easily, flammable material placed over the fuse.

Another object of the invention is to provide in the usual porcelain fuse plug housing having a bottom central contact and a side contact, a con- 25 ductor connecting said contacts within the housing, the lower end being attached to the central contact by an alloy having a lower melting point than that of the conductor, a portion of the conductor near the alloy having a restricted portion which under certain current conditions can generate suflicient heat to melt the alloy, and the upper portion of the conductor being resilient and in the normal condition of the fuse under such tension that when the heat generated in 95 the lower part of the conductor softens the alloy the tension of the upper part of the conductor causes the separation of the lower end of the conductor and the bottom contact. Furthermore the upper tensioned end of the conductor is so constructed that when the lower end breaks away, the severed portion rises to a point under the mica top where it is clearly visible thereunder, and to further this visibility a part of the upper part of the conductor may be slitted out 45 and curved upwardly so that it can move up adjacent the lower wall of the mica window, and may also carry a spot of color thereon to assist visibility thru the mica. A centrally apertured disk of paper, asbestos, or other suitable material 5 may be placed under the mica and so positioned relatively to the upwardly curved part of the conductor that when the conductor is separated from the contact the curved part moves upwards and appears in the aperture of the disk thereby 55 clearly indicating thru the mica the blowing of the fuse.

Other objects of the invention will appear as the description proceeds, and it will be understood that the description is only by way of demonstration of the working of the invention and not in any way is the invention limited thereby.

Reference is made to the drawing in which Figure 1 is a central vertical section showing a side view of the conductor.

Figure 2" is a vertical central section showing a front view of the conductor.

Figure 3 is a top view after fusing.

Figure 4 is a View similar to Figure 1 after fusing.

A porcelain housing ID has a hollow chamber open at the top and screw threads on the sides to receive a metal shell I I threaded to fit into the usual threaded socket of well known construction (not shown). Centrally of the bottom of the housing It is a metal cup socket I2 adapted to hold a suitable solder alloy I3. Secured in the alloy I3 is the lower end of a conducting strip I d, which may be copper, and which has a narrower or restricted section I5 above the alloy so that when electric current is passing thru this section I5 the electrical resistance will be increased and this section will become highly heated and the heat conducted down into the alloy I3, which has a lower melting point than the conductor I4, so that when a current of predetermined magnitude for a predetermined period of time flows thru the conductor I4 the alloy l3 softens and permits the free withdrawal of the conductor end from the alloy.

In order to break electrical contact at the alloy E3 the conductor 14, the opposite end of which is in electrical contact with the shell II by means of a soldered joint I6, is so tensioned that contact is automatically broken at the alloy 53 when the alloy I3 melts. A preferred means of tensioning the conductor I4 is to form the upper portion thereof of resilient metal and to position it so that its normal relationship to the housing ID is as shown in Figure 4 and thereafter to bend down the conductor I4 and to seal the lower end in the alloy I3 as shown in Figure 1, so that the conductor itself is under tension. The conductor I4 may be one continuous strip or may be formed of two parts suitably riveted or otherwise connected at H.

In the upper or transversely extending part of the conductor I4 a strip I8 may be cut out on three sides and bent back over the conductor I4 to expose on top an upwardly extending fiat surface, which surface may carry a spot of bright colored paint or the like. In securing the usual mica disk l9 on top of the housing I0 by the centrally apertured ferrule 2| as is known in the art, a disk 22. which may be paper, Celluloid, or other suitable material is inserted under the mica disk I9. This disk 22 has an aperture 23 so positioned that it exposes the extension I8 in the fused condition as shown in Figure 4, and when this disk 22 is used the color thereof is preferably contrasted with the spot of color on top of the extension I8.

Having shown and described a preferred form of my invention and realizing that, in view of my disclosure, many modifications in form or material, or the substitution or omission of parts, may readily occur to those skilled in the art, I do not limit mvself to the exact form shown or described.

I claim:

1. A delayed action thermal circuit interrupter comprising a cup-shaped insulating body having an internal closed chamber, a threaded conducting shell on the sides of said body, a center contact extending thru the bottom of said body, a fusible metal alloy in the body and on the contact, and a conductor in the chamber connecting said contact and said shell, said conductor having a higher fusing point than said alloy, the lower end being releasably held by said alloy, the conductor including a flat metal strip cutaway on one side to provide a restricted portion adjacent said alloy that increases the electric resistance and generates heat, and the conductor including a tensioned resilient portion operative to withdraw the lower end from contact when said alloy is fused.

2. A delayed action thermal circuit interrupter comprising a cup-shaped insulating body having an internal closed chamber, a threaded conducting shell on the sides of said body, a center con tact extending thru the bottom of said body, a fusible metal alloy in the body and on the contact, and a conductor in the chamber connecting said contact and said shell, said conductor having a higher fusing point than said alloy, the lower end being releasably held by said alloy, the conductor including a flat metal strip having a restricted portion of smaller cross-sectional area than the rest of the conductor and adjacent said alloy that increases electric resistance to generate heat when the current overload becomes dangerous, and the conductor including a tensioned resilient portion operative to withdraw the lower end from contact when said alloy is fused.

3. A delayed action electrical fuse comprising an insulating casing, two separate terminals in the casing, one in the bottom and one in the side of the casing, a conducting element within said casing and connecting said terminals, one end of said conducting element being connected to the bottom terminal by an alloy, said alloy having a lower fusing point than the conductor, and the conductor having a restricted portion near and above said alloy that increases the electric resistance to generate heat which is conducted to the alloy in sufiicient quantity during dangerous current overloads to melt the alloy, the alloy end of the conductor being under tension and tending to break said conducting element away from said alloy terminal when the electric current thru the conductor reaches predetermined magnitudes for predetermined times.

4. A delayed action electrical fuse comprising an insulating cup-shaped casing, two separate terminals in the casing, one in the bottom and one in the side of the casing, a conducting element within said casing and connecting said terminals, one end of said conducting element being connected to the bottom terminal by an alloy, said alloy having a lower fusing point than the conductor, and the lower portion of the conductor being substantially fiat metal and having a restricted portion near and above said alloy, which restricted portion increases the electric resistance to generate heat that is conducted to the alloy in sufiicient quantity during dangerous current overloads to melt the alloy, the portion of the conductor towards the other terminal being re silient and under tension and tending to break said conducting element away from said alloy terminal when the electric current thru the conductor reaches predetermined magnitudes for predetermined times.

LOUIS LUDWIG. 

